Concepts behind plasmid prep - (Mar/24/2010 )

Hello, I was curious to know why a plasmid prep isolates only the plasmid DNA and not the bacterial chromosome? Could someone explain how the different steps of a typical plasmid prep lead to the isolation of only plasmid DNA?

Thank you

The chromosomal DNA is too much "wide spread" it can not rebind (or not in a correct why, it will "reconnect" but not good enough) , the plasmid DNA is not so big and can "reconnect" after denaturing.

Eum I know this sounds strange, a picture might explain it better.

The circles are plasmids, the "lines" are chromosomal DNA.



I hope you understand it?

So in short: you denature both chromomal DNA and plasmid DNA and then you renature it with the acid. And only the plasmid DNA can renature effectivly.

spellberg56 on Mar 24 2010, 04:21 PM said:

To put it another way:

With the most commonly used method (alkaline lysis) chromosomal and plasmid DNA are separated by differential denaturation/renaturing. Bacteria are lysed with ionic detergent (SDS) and NaOH. During lysis both types of DNA are denatured. Subsequent rapid neutralization (by addition of a concentrated potassium acetate buffer) allows only the covalently closed plasmid DNA to reanneal and stay solubilized. Most of the chromosomal DNA and proteins precipitate in a complex formed by an insoluble potassium-SDS complex. This is removed by centrifugation.

Further purification by anion-exchange (eg Qiagen columns) removes further contaminants such as salts and protein. This relies on selective binding/elution of the strongly negative charged DNA to a gel containing positively charged chemical groups.

Hope this helps.